In general, electro-optic (EO) devices such as the THz generator exploit material properties such as electro-optic coefficient, susceptibility, and processing parameters for creating smart and high performance devices. The materials used in the current art are primarily inorganic materials such as lithium niobate (LiNbO3), galium phosphide etc. Other inorganics such as ferroelectric relaxors, [e.g., lead magnesium niobate (PMN), lead zirconium niobate (PZN) and mixtures of these ferroelectric relaxors], and electro-ceramics such as lead lanthanum zirconium titanate (PLZT) have also been mentioned. Zinc telluride (ZnTe) is also commonly used for THz sensing. However, the biggest problem with this class of inorganic materials is that they are not compatible for monolithic lithography and integrated device fabrication. Additionally, they require very high temperature processing, beyond the range of the lithography process. Further, commonly used materials have a smaller EO coefficient; e.g, ˜33 pm/V for LiNbO3 and ˜25 pm/V for ZnTe. Thus, the device performance is limited by material constraints. It has been shown that PMMA with a dye (called Lemke) has over ˜0-3 THz window. This material's reported EO coefficient is 25-30 pm/V. Because of its low dielectric constant, it exhibits a better phase matching and generates a higher power (˜mW) compared to its inorganic rival (ZnTe crystal). However, it has the disadvantage that it has a low glass transition temperature, Tg (˜85° C.), and photo-degradation of the dye molecules from continuous exposure to 510 nm laser occurs. It has also been reported that a polymer based parallel plate metal waveguide in ˜0.5-2 THz window showing that both optical pump beam and generated THz radiation can propagate in the fundamental mode of the waveguide. This allows for a noncritical phase matching over an interaction length of up to 3 mm. They used a 3 μm active core layer composed of 10-mol % disperse red 1-methyl methacrylate (DR1-MMA) copolymer that has a linear chain molecular structure.
The THz region of the spectrum has not been utilized to full potential to harness its power, primarily because of the difficulty in providing a suitable THz source. A limited number of currently available THz sources are usually weak, bulky, expensive, and incoherent. Nonetheless, there are many important reasons why the THz range is of intrinsic scientific interest and it is also rich in potential commercial applications.
THz radiation, also called T-ray, is non-ionizing, penetrates many materials that are opaque to visible and infra-red (IR) light. T-rays also suffer less Rayleigh scattering than infra-red; interact strongly with water, but passes through a few mm of biological tissue and a kilometer of mist. Many material excitations lie in the THz range, e.g. molecular rotations and vibrations, giving rise to a molecular “signature”. It is also noteworthy that a number of natural phenomena involve the THz frequency range: for example, the value of thermal energy, KBT, (where KB is the Boltzmann constant) at room temperature (T=300K) corresponds to approximately 6 THz, so that many biological processes may be regarded as THz phenomena. The advent of molecular signature can be exploited to identify biological molecules for potential applications in medical diagnosis and therapeutics.
Till this day the lack of high power, portable, room temperature THz sources is the most significant limitation of THz systems. Some promising new approaches have the potential to bring THz technology a step further to everyday applications. But many of these sources are still not fully developed technologies. An overview of different THz sources is given in the following paragraphs. THz radiation is naturally emitted by all bodies. The blackbody radiation in this spectral range, below the far infrared, is comparatively weak—lower than 1 μW per cm3. Sources like light bulbs in the visible spectrum are therefore unsuitable as a THz source.
Most techniques providing broadband pulsed THz sources are based on the excitation of different materials by means of ultra short laser pulses. These are photo-carrier acceleration in photo conducting antennas, second order non-linear effects, plasma oscillations, and electronic non linear transmission lines. Unfortunately most of the technologies have very low conversion efficiencies, nano to micro watts, compared to about 1 W power from the optical source. In the photoconduction approach a photoconductor (e.g., GaAs, InP) is shined with ultra fast laser pulses, with photon energy greater than the bandgap of the material, to create electron hole pairs. An electric field of about 10 V/cm is generated in the semiconductor by applying a DC voltage. The applied static field causes the free carriers to accelerate and form a short photocurrent. Because of the acceleration, these moving electrons may radiate electromagnetic waves in the THz range. However, the above mentioned photoconductive emitters are not capable of large average power; a maximum of 40 μW power and a bandwidth of ˜4 THz may be obtained.
Another approach for THz generation is optical rectification. Here again ultrafast laser pulses are used in combination of the non-linear properties of materials. This means that the pulsed optical beam itself is the origin of THz radiation. These nonlinear effects arise when one illuminates a crystalline lattice with higher intensities. The required photon energy is achieved through a down conversion process. This means that an incoming beam splits into two outgoing beams of lower frequencies: ωin=ωO1+ωO2, where, ωin is the angular frequency of the incoming beam and ωO1 and ωO2 are the angular frequencies of the outgoing beams. The output frequency is not unique. Research in this field has focused in the past on materials like GaAs, ZnTe, and organic crystals. This process provides THz radiation only with very low efficiency.
The narrowband sources mechanisms range from upconversion of radio frequency to different kinds of lasers, including gas lasers, free electron lasers, and quantum cascade lasers. One technique to generate a low power continuous wave (CW) THz radiation is through upconversion of lower frequency microwave oscillators. Frequencies up to 2.7 THz have been demonstrated. Another common source is gas lasers. The gases used are mainly methanol and hydrogen cyanide. In this method a CO2 laser pumps a low-pressure gas cavity with one of the above gases, which lases at the gas molecules emission lines. These frequencies are not tunable and require large cavities and high (kilowatts) power supplies with only output power of the magnitude of milliwatts, thus extremely low efficiency.
Another highly awaited source for THz radiation is semiconductor lasers. In the past these lasers had useful applications in industry because of their higher efficiency. Such a compact system for THz radiation is however still missing. This indicates that although silicon is a capable material for electronics, it is not a suitable or capable material for optics and THz generation.
Another approach to build a semiconductor laser in the THz region is based on quantum cascade laser (QCL). A quantum cascade laser consists of periodic layers of two semiconductor materials, which form a series of coupled quantum wells and barriers with a repeating structure. The wells and barriers are usually nanometer thick layers of GaAs between “potential barriers” of Al—GaAs. Quantum confinement within the wells causes the conduction bands to split into a number of distinct sub bands. Light is emitted by transition of electrons from a higher state to a lower state in the well. As the difference between the energy levels is determined by the thickness of the layers, the produced frequency can be chosen by design of the layers.
The quantum cascade lasers operate only at very low temperatures, at liquid nitrogen. The energy spacing between the inter-subbands (ΔE˜0.004 eV) is very small compared to room temperature energy, kBT˜0.025 eV. Since kBT is not lower than ΔE, electrons are always exited into higher subband states. In a quantum cascade laser the subbands couple from the lower state to the higher state of the cascaded quantum wells. If the electrons are now all in upper states they cannot jump anymore through the stairs of the well structure and cannot be used for the lasing. Therefore, room temperature operation is not possible.
It is seen from the above review that there is no turnkey THz source available. A principal barrier to effective application of THz technology in many real-world situations is the available THz power from highly compact sources. For many practical applications, such as inspection and screening, detection of hidden weapons on humans, illicit materials, biological imaging, etc., the lack of power necessitates long signal acquisition times, and is insufficient to divide among an array for parallel detection or synthetic aperture imaging. High power is also critical for sub-surface imaging and spectroscopy applications, where the THz beam may suffer considerable attenuation due to absorption and scattering as it propagates through the medium.